(1) Field of the Invention
This invention is related to an instrument used in the field of opthalmology and plastic surgery and, more particularly, it relates to ophthalmic instruments that are used for measuring the exopthalmos and the enopthalmos of the eyes.
(2) Description of Related Art
As the prior art FIG. 1A illustrates, the eyeball 100 is located in a boney socket (the eye socket) 102 that protects the eyeball 100 from trauma. The space inside the eye socket 102 is generally known as the “orbit,” which contains fat and connective tissues that support the eyeball 100 and its external attachments such as vessels, nerves, muscles, etc. Exopthalmos (or “proptosis”) is the abnormal protrusion of the eyeball 100 from the orbit (anterior displacement) due to exopthalmogenic diseases that cause the swelling of the orbital content, resulting in the protrusion of the eyeball 100 out of the eye socket 102. Enopthalmos is the backward (or posterior) displacement of the eyeball 100 into the orbit, which may be caused by, for example, fracture of the eye socket 102.
Opthalmometers are used extensively to measure the amount of exophthalmia and enophthalmia in patients. Opthalmometry of the eyeball position in the orbit facilitates the diagnosis of exophthalmic and or enophthalmic conditions within patients, which is particularly important for patients having orbital disease that can cause exopthalmogenic or enopthalmogenic conditions. For example, progression of the proptosis in a patient with dysthyroid eye disease could be a sign of activity of the disease and may indicate urgent treatment, while, in the stable phase of the disease accurate measurement of the amount of proptosis is important in making the decision for surgery and planning for the amount of correction.
In general, the eyeball position along the anterior-posterior axis of the eyeball is measured by the distance (or orbital distance) 104 between the lateral orbital rim 106 (the outer edge (or the profile) of the eye socket 102) and the corneal apex 108 (the front surface of the eyeball 100). That is, the distance 104 is measured from the lateral orbital rim 106 to a vertical frontal facial plane 109, which is tangent to the corneal apex 108 and perpendicular to the anterior-posterior axis. It should be noted that although the superior orbital rim (not shown) has also been used as a reference point for opthalmometric measurements (e.g., U.S. Pat. No. 5,379,079 to Kratky), it is generally not as reliable of a reference point for opthalmometric measurements compared to the use of lateral orbital rim 106.
There are generally three types of opthalmometric measurements, including absolute, comparative, and relative opthalmometry. The absolute opthalmometry measures the degrees of exophthalmia and or the enophthalmia, and compares this measurement with a known normal value (e.g., an average on a chart). The comparative opthalmometry measures the exopthalmos and or the enopthalmos, but compares the resulting measurements with a previously taken set of measurements in the same eye. In relative opthalmometry the exopthalmos and or the enopthalmos measurements of the right and the left eyes are compared with each other.
As is illustrated in the prior art FIG. 1B, it is possible to perform opthalmometry to determine the exopthalmos and or the enopthalmos by using an ordinary transparent ruler 110. The base of the rule 110 is placed on the margin of the lateral orbital rim 106 and the corneal apex 108 is visualized through the transparent ruler (with the eye of the patient looking straight ahead), then a reading of the measurement is taken. However, if the ruler 110 is tilted to any one of the sides indicated by the directional arrows 112 or the examiner moves or the line of sight of the examiner is not exactly perpendicular to the ruler 110, the readings of the measured displacement of the eyeball 100 (the orbital distance 104) would not be accurate. This produces an error commonly known as the parallax error, which is the apparent displacement, or difference in the apparent position of the eyeball, caused by actual change (or difference) of position of the point of observation (by the examiner).
One of the most widely used opthalmometric instruments is the HERTEL exopthalmometer, which is used to measure the eyeball position. The HERTEL exopthalmometer is composed of two mirrors at each side that overlap the image of the corneal apex on the image of a scale and allows the examiner to perform the measurements while standing in front of the patient. It is a binocular instrument that rests on each lateral orbital rim and allows an observer in front, with the aid of the mirrors, to view images of the corneal apex of the two eyes as seen in profile, superimposed upon a millimeter scale. A measurement is obtained of the relative distance of the apex of the cornea from a zero reference point, i.e., an imaginary horizontal line in a plane parallel to the front of the patient's face uniting the lateral orbital rims.
Large footplates (or bases) and lack of a mechanism to ensure proper resting of the footplates on the lateral orbital rims makes the HERTEL exopthalmometer prone to error in measuring the eyeball position. As examiner's both hands are engaged, holding the instrument next to the patient's face, it is generally not possible to ensure that HERTEL exopthalmometer footplates are resting properly on the lateral orbital rims on each side. Further more, even if the footplates are properly rested on the lateral orbital rims, there is no mechanism to ensure that they are placed symmetrically at equidistance from each orbital rim. When the HERTEL exopthalmometer is parallel to the coronal plane but is displaced to the left or right, asymmetrical placement of the footplate will result in under-reading in both sides as the medially displaced footplate does not sit properly on the lateral orbital rim. However, if the instrument is not parallel to the coronal plane the result would be under-reading of the medially displaced side only, and the reading of the other side will depend on the direction of the rotation of the instrument at horizontal plane.
Freedom of rotation along the horizontal plane (joining the right and the left lateral orbital rims) is another source of opthalmometric errors when using the HERTEL exopthalmometer. The horizontal rotation of the instrument can occur when the examiner applies less pressure on the left or the right lateral orbital rim to rest the footplates of the HERTEL exopthalmometer, especially if either one of the lateral orbital rims is tender, are asymmetrical, or have asymmetric swelling of soft tissues, or are fractured. The rotation at horizontal plane results in under-reading of the side that HERTEL exopthalmometer is rotated towards, and over-reading of the other side.
It has been found that parallax errors are eliminated when the distance between the lateral orbital rim and the anterior-posterior position of the eyeball (the orbital distance) is approximately about 20 mm. However, when this distance differs, it has been found that the images of the corneal apex and the ruler overlap properly only when the sighting line of the examiner is at the right angle to the frontal plane. In other words, movement of the examiner to the right or left, while looking at the image of the apex of the cornea, introduces parallax error. Given the fact that the above distance varies among different people and considering the practical difficulties of proper placing of the footplates, which can increase or decrease the distance between the footplate and the anterior-posterior axis of the eyeball, there is a considerable chance of parallax error with HERTEL exopthalmometer.
Leudde exopthalmometer (illustrated in the prior art FIG. 1C) is another less widely used opthalmometric instrument. Leudde exopthalmometer is a thick transparent (rectangular-cube) ruler that has two exactly similar scales on two opposing sides, along the length of the rectangular-cube. When Leudde exopthalmometer is placed over the lateral orbital margin, the examiner must view the anterior-posterior positional axis of the eyeball by overlapping the two scales in each side of the instrument by moving to the left or right of the instrument so that the scales on either side overlap and look as a single scale. This way, the line of sighting of the examiner would become perpendicular to the instrument. Regrettably, any inward or outward (lateral movement), or any type of tilting of the instrument would cause parallax error.
The Naugle exopthalmometer is yet another opthalmometer that is similar to the HERTEL exopthalmometer, but it uses the superior and inferior orbital rims for measuring the anterior-posterior positional axis of the eyeball, instead of the lateral orbital rim. In addition, the Naugle exopthalmometer may also be used for measuring most vertical deviations of the eyeball. Although the superior and inferior orbital rims have been used by a few other researchers, studies have shown that they are more asymmetrical and less reliable than lateral orbital rims as reference point for measurement. However, the Naugle exopthalmometer is superior to HERTEL in fractures of orbit that involve lateral orbital rim and in cases for patients that have had surgical removal of the lateral orbital rim.
The British patent 655,787 to Copper discloses an opthalmometric instrument that is used for measuring the resistance encountered in forcing the eyeball back into the orbit or eye socket, with such resistance affording the oculist data concerning the consistency and other qualities of the intra-orbital tissues. The opthalmometric instrument taught by Copper also enables measurement of exopthalmos or protrusion of the eye along the anterior-posterior axial displacement of the eyeball.
The Copper instrument includes two metal strays 3 of seemingly equal length at a right and a left side of the instrument, that couple with a headband at one end (proximal to the eye), and a flat metal strip cross bar 1 at the other end (distal from the eye). The proximal ends of the strays 3 coupled with the headband rest on the right and the left orbital rims, simultaneously, while the distal ends thereof are coupled with the cross bar 1 via a set of nuts 5. The cross bar 1 and the metal strays 3 form a “frame” for the patient to be worn during examination. The use of headband to secure the instrument on a patient's face while the measuring instrument (dynamometer) touches the eyeball is not practical in that the headband itself may slip, tilt, move, or not be placed or oriented properly, causing a possible injury to the eye during examination. Further, the use of seemingly equal length metal strays 3 will result in erroneous measurement of the degree of exopthalmos on patients with asymmetrical lateral orbital rims.
Unfortunately, present constructions of instruments do not provide a particularly accurate opthalmometric measurements (or readings) in that they depend on the position of the person using the instrument (the examiner), and indeed, any slight movement of the examiner, the patient, or the instrument itself can lead to incorrect and unsafe examination of the eyes. Therefore, in light of the current state of the art and the drawbacks to current instruments mentioned above, a need exists for an apparatus and a method that would mitigate these problems. In particular, a need exists for an apparatus and a method that would provide a stable base or frame for opthalmometric measurements, obviate parallax errors, and be safe to use.